This invention relates to an electron gun for generating one or more electron beams and more particularly to an electron gun provided with means for effectively focusing the electron beams on a target.
With the ordinary color picture tube provided with a multi-beam electron gun designed to generate a plurality of electron beams, the respective electron beams pass through separate electron lenses to be focussed at a point on a target. The electron lens is generally formed of a static electric field to focus the electron beams at a single point. The static electric field is formed at right angles to an electron beam path, and is disposed between at least two electrodes each bored with an opening allowing the passage of an electron beam. The properties of the electron lens can generally be varied according to interelectrode voltage, the size of an opening bored in the electrodes and a distance therebetween.
The electron gun is generally regarded to have a more improved performance, according as the electron lens is more reduced in the degree of magnification and spherical aberration. To provide an electron gun of high quality, therefore, it is necessary to extend the focal length of the electron lens. The most effective process to attain this object is to vary interelectrode voltage. However, the level of the interelectrode voltage should generally be restricted to fall within such a range as prevents arcing from taking place at the base portion of a picture tube. Further, enlargement of an electrode opening to extend the focal length of the electron lens is subject to certain limitations, because the neck diameter of the picture tube is restricted by other electrical requirements. Moreover, extension of the interelectrode distance is not advisable since the properties of the electron lens are harmfully affected by a electric charge occurring in the neck portion of the picture tube and the generation of an unnecessary electric field in the electron gun. As mentioned above, the design of the electron lens is subject to limitations due to various physical requirements associated with the construction of a picture tube. These limitation are particularly rigid in the case of a color picture tube using a multi-beam electron gun.
The customary process of manufacturing an electron lens having a long focal length without being obstructed by the above-mentioned limitations is to combine properly interelectrode voltage and the kind of electrode. An electron gun constructed by the above-mentioned process has already beam set forth in the Japanese patent disclosures Nos. 76072/1976 and 77061/1976.
However, the disclosed processes have the drawbacks that the electron gun unavoidably has a complicated construction and extra voltage has to be applied to improve the formation of an electron lens, thus leading to economic disadvantage. For elevation of the performance of an electron lens, it is necessary to apply high voltage with respect to not only the electron guns used in the above-mentioned disclosed processes but also electron guns in general use. In such a case, a special device has to be provided to suppress arcing which might otherwise occur in the base portion of a picture tube in order to ensure its reliable operation, thus rendering the picture tube more expensive.